The invention relates to chemical vapor infiltration techniques. The field of application of the invention is densifying porous substrates, in particular making composite material parts by densifying fiber substrates by means of a matrix.
In conventional manner, a method of densifying substrates by chemical vapor infiltration comprises the steps of loading porous substrates to be densified into a loading zone of an enclosure, heating the internal volume of the enclosure, introducing a reagent gas in the enclosure through an inlet situated at one end thereof, and preheating the reagent gas after it has entered into the enclosure and before it comes into contact with the parts situated in the loading zone.
The temperature and the pressure that exist inside the enclosure are selected so as to enable the reagent gas to diffuse into the pores of the substrates and deposit therein the material for constituting the matrix, either by one or more components of the reagent gas decomposing, or else by a plurality of components reacting together.
The reagent gas is conventionally preheated by passing the gas through a preheater zone situated inside the enclosure and into which the reagent gas inlet opens out. A conventional preheater zone comprises a plurality of perforated plates disposed one above the other and raised to the temperature inside the enclosure.
The purpose of preheating the reagent gas is to ensure that when it enters into the loading zone it is at a temperature that is as close as possible to the temperature required for forming the desired matrix. When the reaction temperature is typically about 1000° C. in order to form a matrix of pyrolytic carbon or of ceramic, having the reagent gas at a temperature that is only a few tens of ° C. below the desired temperature can have a significant effect on the rate of densification and on the microstructure of the deposited matrix material.
This has been observed particularly in the case of densifying substrates disposed in stacks, in particular substrates of annular shape for making brake disks out of composite material. Methods and installations for densifying annular substrates in stacks are described in documents U.S. Pat. No. 5,904,957 and EP 0 792 385. The reagent gas coming from the preheater zone is admitted into the internal volumes of the stacks which are made up of superposed annular substrates and which extend vertically in the loading zone above the preheater zone, with the reagent gas inlet being situated at the bottom of the enclosure. A densification gradient is observed between the substrates situated at the bottoms of the stacks and the other substrates, which gradient becomes greater the more insufficient the preheating of the reagent gas.
The problem could be solved by increasing the volume of the preheater zone. However, for a given total enclosure volume, that would reduce the space available into which substrates can be loaded. Unfortunately, the processes of densification by chemical vapor infiltration are lengthy and expensive to implement, so installations need to have their loading capacities used to the full.
In addition, the reagent gas reaching the tops of the stacks has traveled through them along their full height and has matured, such that the substrates situated at the top of the stacks receive a reagent gas of composition that may be different from that of the reagent gas on entering into the loading zone. This also can give rise to densification characteristics that are different.